The present invention relates to light response enhancement of image sensors.
Due to its many advantages, Complementary Metal Oxide Semiconductor (CMOS) image sensors have been of interest as replacements for charged coupled devices (CCDs) in imaging applications. A CMOS image sensor generally uses a single low power supply and has a simpler system level design with high functional integration when compared with a CCD image sensor. These factors contribute to lowering system costs while providing for a potential camera on a chip. Such features are highly desirable, for example, in camcorders and digital cameras, where the devices may be reduced to a size of a TV remote control and are highly portable. Additionally, high resolution color images can be recorded for hours on battery power because the CMOS image sensor has a low power consumption.
The CMOS image sensor can be generally divided broadly into two categories dependent on the type of pixel array used, the first category being the passive pixel array and the second category being the active pixel array. In the passive pixel array, each pixel merely collects the charge generated by the photodiode and transfers the collected charge to the imaging circuitry for image processing. The active pixel array, on the other hand, includes an amplification circuitry in each pixel to amplify the signal represented by the charge generated by the photodiode before transferring to the image circuitry for processing. The advantage of the passive pixel array over the active pixel array is that each pixel has minimal components allowing for a high fill factor which in turn produces a high quantum efficiency. Fill factor generally refers to the ratio of photo sensitive area to the pixel""s overall size. Quantum efficiency is a measure of light sensitivity and refers to the ratio of photon generated electrons that a pixel captures to the photon incident over the pixel area. However, one of the disadvantages of the passive pixel array is that the charge levels generated may be low and thereby insufficient to drive the image circuitry to produce high quality images. In the active pixel array, the pixel amplifies the signal represented by the charge and is sufficiently able to drive the image circuitry. However, due to several components being used for amplification, the fill factor is generally low which in turn affects the quantum efficiency. The active pixel array generally compensates for the low quantum efficiency by using microlenses to focus the photons into the sensitive area of the pixels that may otherwise strike the insensitive area of the pixels. Microlenses, however, are expensive and generally drives up the cost of manufacturing the active pixel array sensor.
The CMOS image sensor technology is by no means a new technology and both the CMOS image sensor and the CCD image sensor were developed at about the same period. While there were many advantages to using a CMOS image sensor over a CCD image sensor (as described above), the CCD image sensor has prevailed over the CMOS image sensor in imaging applications. One major reason is that the CMOS image sensor has not been able to match the quality of the image generated by the CCD, that is, light sensitivity has been one issue in which the CCD image sensor has prevailed over the CMOS image sensor. The CMOS image sensor, however, is now rapidly gaining wide acceptance due to increased light sensitivity obtained using various complicated and expensive enhancement technologies. However, from a cost point of view, the low cost advantage of the CMOS image sensor has severely eroded when compared with the cost the CCD image device due to the enhancement technologies. It is desired to boost the light sensitivity of a CMOS sensor so that better quality images may be produced and where possible, maintain the cost advantage of the CMOS sensor.
A method and apparatus is described that is related to light response enhancement of image sensors. A phosphor layer is placed between the incident photons and the image sensor in which the phosphor layer converts incident photons from a first wavelength to a second wavelength.
Other features and advantages of the present invention will be apparent from the accompanying drawings and detailed description to be followed.